The invention relates to the storage of data used by a computer.
A computer typically accesses data in files stored on various media having different rates of access associated with them. A computer's solid state disk and local disk often permit the quickest access and would desirably be used for files that are frequently accessed. The computer might store files rarely used on a tape library that is accessed over a computer network.
It is known to provide hierarchical storage management systems to automatically move less used data files to slower storage devices as the faster storage devices fill up beyond set storage levels. Such systems have a goal of "infinite storage." Typically the contents of files are moved from a local disk to an archive medium such as tape, and a special file name is left on the system, sometimes referred to as a "stub." The special file name looks like a normal file name, but, when it is accessed, the hierarchical storage system is informed to retrieve the information from tape and load it back onto the local disk. The I/O requests are then satisfied from the local disk.
The term "logical level" refers to data as seen by the user application programs in files or database tables. The term "physical level" refers to data as stored in "blocks" (there typically are a number of blocks in each file) at specific locations on some physical medium, e.g., a host computer disk. Typically the computer's operating system (e.g., UNIX or DOS) includes a file system that does mapping between the logical level and the physical level.